598 Part VI: Fermented Foods
pounds in cultured dairy products. Many of these
bacteria have lactase activity and hydrolyze lactose
to its monosaccharide units, glucose and galactose,
prior to further metabolism. The hydrolysis of lac-
tose in most cultured dairy products is significant for
individuals who are lactose intolerant, allowing them
to consume dairy products without the undesirable
effects of the inability to hydrolyze lactose.
Homofermentative lactic acid bacteria will pro-
duce lactic acid from these monosaccharides, ac-
cording to the reaction
Lactose 4 ADP 4 H 3 PO 4 →4 Lactic Acid 4
ATP 3H 2 O
The glucose and galactose molecules are metabo-
lized through the glycolytic and tagatose pathways,
respectively. Key steps in the formation of lactic
acid from the monosaccharides include the hydroly-
sis of the hexose diphosphates to glyceraldehyde-3-
phosphate by aldolases, the formation of pyruvate
from phosphoenol pyruvate by pyruvate kinase, and
the reduction of pyruvate to lactate by lactate dehy-
drogenase.
Heterofermentative lactic acid bacteria produce
carbon dioxide, acetic acid, and ethanol, in addition
to lactic acid during the metabolism of the monosac-
charides, according to the reaction
Lactose 2 H 3 PO 4 2 ADP →2 Lactic Acid 2
Ethanol 2 CO 2 2 ATP H 2 O
The heterofermentative lactic acid bacteria lack
aldolases; therefore, the sugars are metabolized
through 6-P-gluconate pathway rather than the gly-
colytic pathway. In this pathway, glucose-6-phos-
phate is oxidized to 6-phosphogluconate, which is
decarboxylated to a pentose-5-phosphate and carbon
dioxide by phosphoketolase. The pentose-5-phos-
phate is converted to glyceraldehyde-3-phosphate,
which enters the glycolytic pathway to form lactic
acid and acetyl phosphate, which is metabolized to
acetaldehyde. The acetaldehyde is reduced to eth-
anol by alcohol dehydrogenase. However, several
bacteria, including Lactococcus lactis ssp. lactis
biovar. diacetylactis, Streptococcus thermophilus,
and Lactobacillus delbrueckiissp. bulgaricuslack
dehydrogenase and accumulate acetaldehyde.
The production of acid by the lactic acid bacteria
has a significant impact on the safety and quality of
the cultured dairy products. The reduction in pH
increases the shelf life and safety of the fermented
dairy products through the inhibition of spoilage and
pathogenic microorganisms.Acid production by these
lactic acid bacteria is critical for the precipitation of
the casein proteins in the formation of several cul-
tured dairy products, such as yogurt, sour cream, and
unripened cheeses. These bacteria may also con-
tribute to the degradation of proteins and lipids
through proteolytic and lipolytic reactions to further
develop the unique texture and flavor characteristics
of the cultured dairy products. These reactions are
especially important in ripened cheeses. The selec-
tion of the appropriate type and level of starter cul-
ture is imperative for the development of the appro-
priate flavor characteristics.
Many commercial dairy processors now use the
direct vat inoculation (DVI) process for frozen or
freeze-dried cultures (up to 10^12 bacteria per gram of
starter) in the processing of fermented dairy prod-
ucts. The use of DVI cultures allows the dairy pro-
cessor to directly add the cultures to the milk and
bypass on-site culture preparation. This recent pro-
gress in the development of starter cultures has also
increased phage resistance, minimized the formation
of mutants that may alter the characteristics of the
starter cultures, enhanced the ability to characterize
the composition of the cultures, and improved the
quality consistency of cultured dairy products. How-
ever, the DVI process is limited by the additional
cost of these cultures, the dependence of the cheese
plants on the starter suppliers for the selection and
production of the starters, and the increased lag
phase of these cultures in comparison with that of
on-site culture preparation (Stanley 1998, Tamime
and Robinson 1999a, Canteri 2000).COAGULATION OFMILKPROTEINSThe production of lactic acid by LAB decreases the
pH of the milk to cause coagulation of the casein. As
the pH decreases to less than 5.3, colloidal calcium
phosphate is solubilized from the casein micelle,
causing the micelles to dissociate and the casein
proteins to aggregate and precipitate at the isoelec-
tric point of casein (pH 4.6). The resulting gel,
which is somewhat fragile in nature, provides the
structure for sour cream, yogurt, and acid-precipitated
cheeses, such as cream cheese and cottage cheese.
The casein micelles are also susceptible to coagu-
lation through enzymatic activity. Rennet, a mixture
of chymosin and pepsin, obtained from calf stom-